This invention relates to circuit testers particularly, but not exclusively, to circuit testers for locating faulty or incorrectly fitted bulbs in a series-wired string of Christmas fairy lights.
Most strings of fairy lights presently in use are series-wired and are powered by domestic mains electricity from a wall socket. The strings usually consist of sets of 50 2.2 v (nominal) fairy light bulbs wired in series and connected by various arrangements to a domestic electricity supply.
Almost all fairy light bulbs in manufacture now have an internal metal oxide coated xe2x80x9cshuntxe2x80x9d wire which is wrapped around the support terminals of the bulb filament. When the filament fails and the string becomes an open circuit, the full household voltage, typically 110 v in the USA, appears across the failed filament. This high voltage breaks down the insulating properties of the oxide coating and the xe2x80x9cshuntxe2x80x9d instantaneously conducts the full load current, and in doing so welds itself to the support terminals. This effectively bridges the open circuit left by the faulty bulb filament and enables the rest of the lights in the string to continue to work.
However, sometimes, these shunt wires fail and the open circuit remains open and at other times the wire conductors coming from the bulb break, in each case providing an open circuit and a dead light string. Similarly, these conductors may not make proper connections with the brass contacts inside the bulb socket, or the bulb itself may shake loose inside the socket, losing contact therewith.
The net effect of these various occurrences is to cause a very visible 50 bulb outage of the whole light string. Finding where the fault lies can be very time consuming and frustrating as it generally requires a substitution test with a known good bulb. Each bulb in the dead string needs to be swapped with the good bulb until the bad bulb is found, and should there be two or more bad bulbs in the same group of 50, substitution testing can be fruitless.
The Christmas fairy lights string industry has come up with several kinds of circuit testers for locating faulty light bulbs. These mostly rely upon an electrical sensing plate which senses the presence of an AC current field around the bulb. In general, a sensing plate is coupled to a high-gain amplifier which in turn activates an LED indicator in the presence of the AC field. Careful selection of components enable these devices to be adjusted so that the user can tell when there is a current in the region of the bulb, by the illumination of a single LED indicator.
However, the instructions for these kinds of devices describe a very complex means of conducting the required test in that it is first of all necessary to establish the active phase wire in the string and ensure that it appears on the bulb nearest the wall plug. Then it is necessary to methodically test each bulb up the string until the LED indicates a marked difference in response to the previously tested bulb. Even then, it is necessary to change that particular bulb and the bulbs on either side because such testers sense any field and hence they will sense the field in each bulb that works including the bad one because it still has an active wire going into it.
Such testers therefore do not discriminate between the two wires and cannot tell if the electric field is present on the outgoing wire as well as the incoming one.
This procedure is also much more complicated in practice as manufacturers loop the 50 bulb light strings back on themselves when they twist them into a single string, meaning that electrically consecutively wired bulbs are not necessarily next to each other in the string. The bulb nearest the wall plug may have the last bulb in that group of 50 next to it, therefore adding to the complexity of testing by using such devices. In addition, where groups of such strings are powered from the same socket extra wires have to be run through the initial string to activate the next string and any external xe2x80x9cadd-onxe2x80x9d sockets for operating e.g. rotatable Christmas ornaments. As a result, the electrical energy being radiated by these additionally active wires can result in very ambiguous and frustrating readings being picked up by such testers which rely, as aforesaid, on simply detecting the presence of an AC field.
The foregoing disadvantages are partly solved by the teaching of U.S. Pat. No. 5,047,721 issued to Farley, the disclosure of which is incorporated herein by reference, which describes an apparatus and method utilising a differential sensing system where the light bulb of a fairy light string is placed into a cavity between a pair of electrodes (60), (62) embedded in the end of a probe (44). This arrangement provides a system for sensing differential AC fields at the bulb socket but requires the user to align the internal contacts of the bulb socket with the sensing plates or electrodes in order for an accurate reading to be taken. If the position is not perfectly aligned an ambiguous reading will be obtained as confirmed in the paragraph commencing at line 25 in column 5 of that patent. Necessarily, as the internal contacts of the light socket and bulb are not visible to the user, a careful explanation has to be given as to how to go about aligning these internal contacts and, in practice, it has been found that many potential users of such a device would not be prepared to devote time and attention to such a procedure.
Also, as a general comment, there are hundreds of different shaped bulbs on the market and hence using a universal sized cavity of light tester as shown in U.S. Pat. No. 5,047,721 will clearly not work in all cases.
A further disadvantage in pursuing the xe2x80x9clight socketxe2x80x9d approach to bulb testing or testing the integrity of the contacts between bulbs and bulb sockets etc is that, as is described in U.S. Pat. No. 6,257,740 issued to Gibboney, other types of bulb socket may include a spring loaded switch to ensure that if a bulb fails by not being seated properly in the light socket then the spring loaded switch ensures that the series circuit remains complete so that other bulbs in the string still remain illuminated. Although this disclosure shows that the spring loaded switch is perfectly symmetrical within the light socket, in practice this is almost never possible and hence different AC fields may be generated, again giving ambiguous readings.
A further disadvantage for all prior art testers stems from the fact that as the user often holds the bulb between fingers of a hand, the fingers themselves become an antenna that gives false indications to nearby capacitance sensors.
The present invention is derived from the realisation that it would be preferable to have the ability to test the integrity of a series-wired circuit, such as a fairy light string, which does not rely upon the need to have to test the circuit at the light bulb or light bulb socket but instead tests between the wires leading to such sockets, either directly adjacent the sockets or at positions along the light string remote therefrom.
According to the invention there is provided an electrically operable circuit tester for testing circuit integrity in a series-wired string of Christmas fairy lights powered by domestic mains electricity, the tester comprising or including a housing; a probe including a pair of matching current sensors protruding from the housing, the probe being adapted to be positioned between a respective pair of wires extending from a fairy light bulb socket, each sensor in the probe being electrically connected to a respective one of a matching pair of signal amplifiers to provide a pair of amplified signals, one for each sensor; a logic gate for receiving amplified signals output from the pair of signal amplifiers and comparing said signals for matching characteristics, the gate providing an output signal to a first display on or in the housing indicative of received and matching signals and a, different, output signal to a second display on or in the housing indicative of received mismatching signals from the amplifiers, the latter thereby indicating a discontinuity in that part of the circuit being tested between the pair of wires, such as by a broken fairy light bulb in that part of the circuit therebetween.
This arrangement provides a significant advantage over prior art devices such as that disclosed by Farley in that the sensors, which may typically be in the form of sensing plates, each act as a shield between the wire adjacent thereto and the other sensor, thereby reducing the possibility of stray signals being picked up from that wire by the other sensor. In contrast, where such sensors as disclosed in the prior art are disposed e.g. on opposing sides of the lamp socket, then each sensor is still able to sense the two electrical fields from both wires, thereby masking out the difference between the two signals and hence providing a lesser differential signal.
Preferably, the circuit tester is battery powered and includes a green LED as the first display and a red LED as the second display. Most preferably the current sensors are disposed symmetrically with respect to each other and are selectively adapted to be disposed between the pair of wires adjacent to the fairy light bulb socket on a Christmas light string.
The logic gate may conveniently comprise an Exclusive Or (XOR) gate or it may be part of a programmed microprocessor. A buzzer may be used to indicate the receipt of mismatched signals which may be in addition to the second display if the latter is a visual display, or the buzzer itself may take the form of the second display, being an audio display as opposed to a visual display.
The invention also extends to a method of testing a series wired circuit including the steps of placing two matching ac current field sensors symmetrically between a pair of wires forming part of the circuit to be tested, amplifying first and second current signals from both sensors and feeding them to a logic gate to compare both signals and provide a third signal in response thereto, said third signal being HI if, but only if, only one of the first and second signals is low and the other is high; and subsequently inverting said HI signal to a low signal to thereafter provide a potential across an electrically powered alarm device, such as a visual display or audio alarm, thereby to indicate a discontinuity in the circuits between the sensors, such as due to a faulty light bulb.